The invention relates generally to cellular mobile communication systems and, more particularly, to a cellular mobile communication system capable of dynamically adapting a power level tiered cell based upon operating conditions within the cell.
A cellular mobile communications system uses a large number of low-power wireless transmitters to create xe2x80x9ccellsxe2x80x9dxe2x80x94the basic geographic service area of a wireless communication system. As mobile users travel from cell to cell within a system, their conversations are xe2x80x9chanded offxe2x80x9d between cells in order to maintain seamless service. The size of cells tends to vary, depending on factors such as subscriber density, demand for services and terrain within a particular region. Typically, rural areas are served by one large cell which operates at a high power level while urban areas are served by many smaller cells, each of which operates at a lower power level.
Because of the limited number of radio channel frequencies available for wireless communications, cellular systems have adopted the concept of xe2x80x9cfrequency reuse.xe2x80x9d The concept of frequency reuse is based upon assigning to each cell a selected group of radio channels for use within a small geographic area. More specifically, each cell is assigned a group of channels which are completely different from the groups of channels assigned to those neighboring cells for which interference between users is possible. The same group of frequencies is then reused in other cells that are far enough away such that interference between users is unlikely. As a service area becomes full of users, a cell is often split into a number of smaller cells.
As demand for wireless communication services grows, the size of cells tends to shrink. As a result, in heavy traffic areas, two users assigned to separate cells but transmitting at the same frequency may be relatively close to each other. In such conditions, interference between the users becomes increasingly likely. Thus, in order to both reduce interference between users of cellular mobile communication systems and to permit more users onto the system, there have been continuing efforts to reduce the amount of RF power needed to maintain contact between a mobile telephone and a base station. One such effort has involved the development of cell tiering.
Cell tiering involves the splitting of a cell in a cellular mobile communication system into an inner tier and an outer tier. Power levels used by the mobile terminals and base station radio transceivers assigned to the inner tier is less than the power levels used by the mobile terminals and base station radio transceivers assigned to the outer tier. Separating the inner and outer tiers is a tier boundary hereafter referred to as a mobile proximity threshold (or xe2x80x9cMPTxe2x80x9d). The MPT may be an attenuation code ranging between 0 and 10 which indicates the power level of the mobile terminal. Mobile terminals in the inner tier are classified as having a moderate probability of interference (or xe2x80x9cMPIxe2x80x9d) while mobile terminals in the outer tier are classified as having a low probability of interference (or xe2x80x9cLPIxe2x80x9d).
A careless setting of the MPT can dramatically affect the rate of noisy and/or dropped calls. In high traffic cells, setting the MPT and monitoring call quality to ensure that the MPT is properly set has proven to be a difficult task. Furthermore, even if the MPT has been properly set, changing conditions within the cell may necessitate a change in the MPT. For example, many base station radio transceivers operate dynamically. A dynamic base station radio transceiver monitors channel interference in real-time and, based upon the detected level of channel interference, automatically determines to which tier the base station radio transceiver is assigned. If one or more base station radio transceivers within a cell automatically switches tiers, for example, by switching from the outer tier to the inner tier upon detecting an increased level of channel interference, the cell may be left with an insufficient number of base station radio transceivers in the outer tier to handle all of the mobile terminals within that tier requesting service. As a result, call quality would be lowered due to the increased number of dropped calls while potential call revenue would remain unrealized.
It is, therefore, the object of this invention to provide a cellular mobile communications system capable of adjusting the tier boundary of a power level tiered cell thereof based upon changing operating conditions therein. By adjusting the tier boundary for the cell, the likelihood of a radio transceiver being available to serve a mobile terminal assigned to one of the tiers is increased and the likelihood of the mobile terminal being dropped is diminished.
In accordance with one embodiment, the present invention is of a power level tiered cellular telephone system having a controller and a base station which includes a plurality of radio transceivers. Each of the radio transceivers is configured for serving mobile terminals within the cell and are initially assigned to either a first or second tier of the cell. When a mobile terminal operates within the cell, the mobile terminal is first assigned to one of the first or second tier based upon a power level for the mobile terminal relative to a power level threshold boundary and thereafter serviced by one of the radio transceivers assigned to that tier. The controller periodically shifts the power level threshold boundary defining an outer peripheral edge of the first tier of the cell based upon a change in an operating condition within the cell. By shifting the power level threshold boundary, certain mobile terminals which would have previously been assigned to one tier are instead assigned to the other. In one aspect of this embodiment of the invention, the operating condition for which a change results in a shifting of the power level threshold boundary is a ratio of the number of radio transceivers serving the first tier to the number of radio transceivers serving the cell. In an alternate aspect thereof, the operating condition is a ratio of the number of channels serving the first tier and the number of channels serving the cell. In another aspect of this embodiment of the invention, the radio transceivers are dynamic radio transceivers capable of changing the tier which they serve.
In another embodiment, the present invention is of a power level tiered cellular telephone system for serving mobile terminals. The cellular telephone system provides service for a cell divided into an inner tier and an outer tier. Mobile terminals operating in the outer tier experience lower levels of interference than those operating in the inner tier. The cellular telephone system includes a base station and a mobile switching center coupled to the base station. The base station includes a plurality of dynamic radio transceivers and an antenna array coupled to each of the dynamic radio transceivers. The dynamic radio transceivers are initially assigned to a selected one of the inner and outer tiers of the cell and initially serve mobile terminals operating within the tier to which they have been assigned. Thereafter, the dynamic radio transceivers can independently switch the tier for which they serve mobile terminals. The cellular telephone system further includes a processor subsystem on which a first software module executes. The software module is configured to assign each one of the mobile terminals to a selected dynamic radio transceiver based upon a power level for the mobile terminal when the mobile terminal initiates operations within the cell relative to a power level threshold boundary which defines an outer peripheral edge of the inner tier of the cell. The first software module also periodically re-determines the power level threshold boundary defining the outer peripheral edge of the inner tier of the cell each time one of the radio transceivers switches from serving mobile terminals operating in one tier of the cell to serving mobile terminals operating in the other tier of the cell. In one aspect of this embodiment of the invention, the cellular telephone system further includes a memory subsystem, accessible by the first software module, for maintaining a list of the dynamic radio transceivers, an indicator as to which tier each one of the dynamic radio transceivers serve and an indicator as to available channels for each one of the dynamic radio transceivers serving the inner tier or serving the outer tier.
In a further aspect of the invention, the cell is divided into at least two sectors, each having a sector inner tier, a sector outer tier and a sector power level threshold boundary which defines an outer peripheral edge of the sector inner tier. In this aspect, the antenna array comprises at least two antennas, each providing coverage for a corresponding sector of the cell. Each one of the radio transceivers are assigned to a selected one of the at least two sectors and initially serve mobile terminals in either the inner tier or the outer tier for the sector to which they are assigned. In a still further aspect thereof, the software module periodically re-determines the sector power level threshold boundary which defines the peripheral edge of the inner tier for one of the at least two sectors of the cell each time one of the radio transceivers assigned to that sector switches from serving mobile terminals operating in one of the sector inner tier and the sector outer tier to serving mobile terminals operating in the other one of the sector inner tier and the sector outer tier.
In still another embodiment, the present invention is directed to a method of adaptively power tiering a cell of a cellular telephone system. In accordance with the method, a value for a mobile proximity threshold boundary is determined. The cell is then separated into first and second tiers with the mobile proximity threshold boundary defining a peripheral edge of the first tier. The value of the mobile proximity threshold boundary is periodically re-determined and, each time the value of the mobile proximity threshold boundary changes, the first tier of the cell is re-sized. In one aspect thereof, the value for the mobile proximity threshold boundary is re-determined upon detecting a change in an operating condition for the cellular telephone system. In another, each one of the dynamic radio transceivers are assigned to serve mobile terminals operating in a selected one of the first and second tiers. In this aspect, the value for the mobile proximity threshold boundary is re-determined each time one of the dynamic radio transceivers changes from serving mobile terminals operating in one of the first and second tiers to serving mobile terminals operating in the other of the first and second tiers.
In still yet another embodiment, the present invention is directed to a method of adaptively power tiering a cell of a cellular telephone system. A plurality of radio transceivers is provided. The cell is divided into first and second parts, each having an area. Each one of the plurality of radio transceivers are assigned to a selected one of the first and second parts of the cell. Each one of the plurality of radio transceivers assigned to the first part of the cell serves mobile terminals operating in the first part of the cell and each one of the plurality of radio transceivers assigned to the second part of the cell serves mobile terminals operating in the second part of the cell. The area of the first part is adjusted each time one of the plurality of radio transceivers serving mobile terminals operating in one of the first and second parts switches to serving mobile terminals operating in the other one of the first and second parts.
In one aspect thereof, a ratio of the number of radio transceivers assigned to the first part of the cell to the area of the first part of the cell is determined. In a further aspect thereof, the area of the first part is adjusted to maintain the ratio of the number of radio transceivers assigned to the first part of the cell to the area of the first part of the cell each time one of the plurality of radio transceivers serving mobile terminals operating in one of the first and second parts switches to serving mobile terminals operating in the other one of the first and second parts.
In still another alternate aspect thereof, the first tier of the cell is an inner tier and the second tier of the cell is an outer tier. In this aspect, a ratio of the number of radio transceivers assigned to the inner tier of the cell to the number of radio transceivers in the plurality of radio transceivers and a ratio of the square of the radius of the inner tier to the square of the radius of the cell are determined. In a further aspect of this embodiment of the invention, each time one of the plurality of radio transceivers serving mobile terminals operating in one of the inner and outer tiers switches to serving mobile terminals operating in the other one of the inner and outer tiers, a ratio of the number of radio transceivers assigned to the inner cell to the number of radio transceivers in the plurality of radio transceivers is re-determined. The radius of the inner tier is then re-determined such that the ratio of the square of the radius of the inner tier to the square of the radius of the cell remains constant. The power tiered cell is then re-tiered in accordance with the re-determined radius of the inner tier of the cell.
In yet another embodiment, the present invention is of a method of servicing mobile terminals operating within a power tiered cell which includes an inner tier and an outer tier using a plurality of communication channels. Each one of the plurality of communication channels are assigned to either the inner tier or the outer tier. A determination is then made as to whether a first mobile terminal to be served is operating in the inner tier or the outer tier. If the first mobile terminal is operating in the inner tier, service to the first mobile terminal is provided over an available channel of the channels assigned to serve mobile terminals operating in the first tier. If, however, the first mobile terminal is operating in the outer tier, service to the first mobile terminal is provided over an available channel of the channels assigned to serve mobile terminals operating in outer tier. When a switch of one of the channels assigned to serve mobile terminals operating in a first one of the inner and outer tiers to serving mobile terminals operating in the other one of the inner and outer tiers is detected, the inner tier is re-sized such that a ratio of the area of the first tier to the area of the cell matches a ratio of the number of channels serving mobile terminals operating in the inner tier to the number of channels serving mobile terminals operating in the cell. A determination is then made as to whether a next mobile terminal to be served is operating in the inner tier or the outer tier. If the next mobile terminal is operating in the inner tier, service to the next mobile terminal is provided over a next available one of the channels assigned to serve mobile terminals operating in the inner tier. If, however, the first mobile terminal is operating in the outer tier, service to the first mobile terminal is provided over a next available one of the channels assigned to serve mobile terminals operating in the outer tier. By re-sizing the inner tier, the inner and outer tiers maintain a constant density of assigned channels per unit area.
In one aspect of this embodiment of the invention, to determine whether a first mobile terminal to be served is operating in the inner tier or the outer tier, a power level for the first mobile terminal is compared to a power level threshold boundary which defines an outer peripheral edge of the inner tier and, based upon this comparison, the tier in which the first mobile terminal operates is determined. In another aspect thereof, to re-size the inner tier such that a ratio of the area of the inner tier to the area of the cell matches a ratio of the number of channels serving mobile terminals operating in the inner tier to the number of channels serving mobile terminals operating in the cell, the power level threshold boundary which defines the peripheral edge of the inner tier is adjusted such that the ratio of the area of the first tier to the area of the cell matches the ratio of the number of channels serving mobile terminals operating in the inner tier to the number of channels serving mobile terminals operating in the cell. In still another aspect thereof, to determine whether a next mobile terminal to be served is operating in the inner tier or the outer tier, a power level for the next mobile terminal is determined. The determined power level for the next mobile terminal is then compared to the adjusted power level threshold value to determine whether the next mobile terminal is operating in the first tier or the second tier.